The present invention relates to the cooling of fluids, and applies particularly to the liquifying of natural gas. It concerns in the first place a process for cooling a fluid, especially for liquifying natural gas, of the incorporated integral cascade type, in which a coolant mixture composed of constituents of different volatilities is compressed in at least two stages and after at least each of the intermediate compression stages the mixture is partially condensed, at least some of the condensed fractions, as well as the high pressure gas fraction being cooled, then being depressurised, put into a heat exchange relation with the fluid to be cooled, and then compressed again.
The pressures dealt with below are absolute pressures.
The liquifying of natural gas using a cooling cycle called "incorporated cascade" utilising a mixture of liquids has long been proposed.
The coolant mixture is constituted by a certain number of fluids which include, among others, nitrogen and hydrocarbons such as methane, ethylene, ethane, propane, butane, pentane, etc.
The mixture is compressed, liquified then supercooled at the high pressure of the cycle which generally lies between 20 and 50 bars. This liquifying can be put into effect in one or several stages with the condensed liquid being separated at each stage.
The liquid or liquids obtained is or are, after supercooling, depressurised to the low pressure of the cycle, generally lying between 1.5 and 6 bars, and vaporised in counter current with the natural gas to be liquified and the cycle gas to be cooled.
After reheating to about ambient temperature, the coolant mixture is once again compressed to the high pressure of the cycle.
For the operation to be possible it is necessary to have available a fluid capable of condensing at ambient temperature at the high pressure of the cycle. This poses a particular difficulty because the mixture and the pressures are generally optimized for the cold part of the liquifying installation and do not lend themselves well to a cooling which performs equally well in the hot part, that is to say lying between the ambient temperature (generally of the order of +30.degree. C. to +40.degree. C. in natural gas production regions) and an intermediate temperature of the order of -20.degree. C. to -40.degree. C.
Thus numerous existing installations require, for the hot part, a separate cooling cycle of propane or a propane-ethane mixture. Thus a relatively low consumption of specific energy is obtained, but at the price of a large increase in the complexity and cost of the installation.